1. Field of the Invention
The present invention relates to a dot position measurement method, a dot position measurement apparatus, and a computer readable medium, and more particularly to dot position measurement technique suitable for measurement of a deposition position of a dot recorded by each nozzle of an inkjet head.
2. Description of the Related Art
One method of recording an image onto a recording medium such as recording paper is an inkjet drawing method in which an image is recorded by ejecting ink droplets in response to an image signal and causing the ink droplets to impact on the recording medium. As an image forming apparatus which employs such an inkjet drawing system, there exists a full-line head image drawing apparatus, in which an ejection unit (nozzle) which ejects ink droplets, is disposed in a line facing the whole of one side of the recording medium, and the recording medium is conveyed in a direction orthogonal to the ejection unit so as to record an image over the whole area of recording medium.
By conveying the recording medium without moving the ejection unit, the full-line head image drawing apparatus is able to draw an image over the whole area of the recording medium and increase the recording speed.
However, with line-head image forming apparatuses, there is the problem that streaks or unevenness of the image recorded on the recording medium occurs due to inconsistencies during production such as displacement of the ejection unit. Such streaks and unevenness are caused by scatter of the ink droplet impact position, and techniques to correct streaks and unevenness, based on the impact position, are known.
Japanese Patent Application Publication No. 2008-44273 discloses a technology whereby a line pattern and, at the same time, a reference pattern are read with a scanner, and the impact position is measured while correcting any scanner conveyance errors.
Japanese Patent Application Publication No. 2008-80630 discloses a technology which reads a line pattern with a scanner to determine the edge position of a line from the read image, and measure the line position (impact position) from a plurality of edge positions for each line.
A large number of commercially available scanners repeatedly execute data transfer and reading, rather than not reading an entire reading range at a fixed speed. Here, a read operation may be suspended and the carriage halted, and the carriage may be operated once again. Although dot deposition position accuracy on the order of 10 μm is a reasonable expectation, when positional accuracy at the submicron level is required, any variation in position caused by the carriage restarting is a cause of errors that cannot be overlooked.
Furthermore, when the measurement target is long in the sub-scanning direction (varies depending on the device type, but roughly 10 cm or longer, only as a guide for example), errors are also caused by a change in position due to wobble of the carriage of the scanning mechanism. Such errors are significant in cases where a line pattern, obtained by arranging lines of deposition dots from adjacent nozzles in different positions in the sub-scanning direction, is measured, as illustrated in FIG. 28.
Line block 0 illustrated in FIG. 28 is a line group block formed by nozzles with nozzle numbers “4N+0” (where N is 0 or a higher integral number), such as the nozzle numbers 0, 4, and 8, when nozzles are assigned the numbers 0, 1, 2, 3, . . . starting at one end of the line head. Line block 1 is a line block of nozzle numbers “4N+1” such as nozzle numbers 1, 5, 9, . . . . Line block 2 is a line block of nozzle numbers “4N+2”, and line block 3 is a line block of nozzle numbers “4N+3”. Thus, lines corresponding to all the nozzles can be formed according to a line pattern in which line blocks, formed with lines using a fixed nozzle pitch, are disposed in different positions on a recording paper 16.
FIG. 29 illustrates the relationship between measurement positions when the scanner sub-scanning position varies. As illustrated in FIG. 29, the measurement positions of line blocks A and B, disposed in different positions in the sub-scanning direction, when line blocks A and B are each measured, are subject to a linear relationship. Scanner-induced errors, as described earlier, appear as disruption of the lattice co-ordinate system read with the scanner.
FIG. 30 illustrates the result of measuring the position (dot position) errors of each line from a line pattern, in which line blocks with a 16-nozzle pitch are disposed in different positions in the sub-scanning direction, instead of the 4-nozzle pitch line blocks illustrated in FIG. 28.
The position errors of the respective nozzle positions are probably random. However, as illustrated in FIG. 30, generally, regular position errors with a 16-nozzle cycle are generated. This includes offset position errors in each of the line blocks in different positions in the sub-scanning direction.
In other words, even though measurement accuracy may be achieved between the data in each of the plurality of line blocks divided in the sub-scanning direction, because a certain offset error is applied for measurement accuracy between the line blocks, a phenomenon arises whereby the measurement result is repeated with similarity, in a cycle containing a number of line blocks.
An error of around 2 to 3 μm for the scanner resolution (2400 DPI, for example) is not problematic in normal case illustrate ever, in cases where measurement on the submicron order is targeted, this deviation cannot be disregarded, and may be a problem when merging the measurement results of the plurality of line blocks.
Furthermore, in addition to scanner-induced errors, similar phenomena are also produced by paper deformation (as an example, for example, in a printing apparatus in which ink is deposited after applying a treatment liquid to recording paper, similar phenomena may occur due to a difference in the extension of the recording paper in the print start position and print end position). In dot deposition position measurement performed with in the presence of paper deformation, similar phenomena can occur due to the combination of both an offset error and a line pitch extension error.
A technology to counter this problem, which corrects the disruption of the image data read by the scanner, is not disclosed or suggested in Japanese Patent Application Publication Nos. 2008-44273 and 2008-80630.